As will be commonly known to persons skilled in the art, electronic drivers have been developed for driving light sources such as gas discharge lamps, LEDs, OLEDs, etc. Such driver is powered from mains, and provides an output current for the light source. The driver may be designed for controlling the current magnitude, but may also be designed for controlling the output power. The driver may have a user control input, typically wirelessly coupled to a remote control, via which a user may control the light intensity, i.e. dim the light source. In such case, dimming is performed by the driver, by reducing the output current intensity (amplitude) or reducing the PWM duty cycle of the lamp current.
There are, however, also situations where the driver does not receive normal mains, i.e. a sine-shaped voltage of for instance 230 V 50 Hz in Europe, but receives a dimmed input voltage. Such situation may typically occur when replacing an existing light source by a light source with integrated electronic driver, when the existing light source is for instance an incandescent lamp powered via a wall-mounted dimmer. Such dimmers typically operate on the basis of phase-cutting the mains voltage, using a TRIAC. Since such dimmers are commonly known, the following description will be kept brief.
FIG. 1A is a graph schematically showing voltage (vertical axis) versus time (horizontal axis) of rectified mains. In can be seen that this voltage follows a continuous sine-shaped curve, of which the negative portions are inverted. The power provided to a resistive load, expressed as P=U2/R, can be considered as being proportional to the surface area under the curve.
FIGS. 1B and 1C are comparable graphs showing the output voltage of phase-cutting dimmers, i.e. a leading edge dimmer (FIG. 1B) or a trailing edge dimmer (FIG. 1C). In the case of a leading edge dimmer (FIG. 1B), the output voltage is suppressed to remain zero immediately after a zero-crossing of the mains, until a certain phase pX between 0 and 180° when the voltage makes a jump to follow the mains curve. Again, the power provided to a resistive load can be considered as being proportional to the surface area under the curve: it can be seen that this power is reduced when said phase pX is increased (righthand side of the curve). In the case of a trailing edge dimmer (FIG. 1C), the voltage follows the mains after a zero-crossing until a certain phase pY between 0 and 180° when the voltage is suppressed to make a jump to zero. Again, the power provided to a resistive load can be considered as being proportional to the surface area under the curve: it can be seen that this power is reduced when said phase pY is decreased (lefthand side of the curve). The situation of FIG. 1B is indicated as “leading edge dimming” and the situation of FIG. 1C is indicated as “trailing edge dimming”.